This invention generally relates to offshore processing facilities and methods for economically conducting the working of offshore petroleum reserves. More particularly, the invention relates to a mobile offshore platform which has a support structure that rests on the ocean floor and a separate removable barge section that is supported by the structure, permitting the use of various barges with the support structure to provide the functions of exploration drilling, production drilling, production, petrochemical processing, and well servicing to be accomplished as required during the reservoir lifetime.
The need for additional oil, gas and other mineral resources has in recent years brought about increased activity in the exploration for and the recovery of such resources from offshore locations. At locations having substantial oil and gas reserves, the approach taken in recovering the minerals has been to erect a permanent platform at the proposed well site and lay pipelines between the platform and the shore to transport the oil and gas to onshore storage or processing facilities.
In erecting a permanent platform, an understructure is brought to the offshore location by transporting the structure to the well site on floats, or on its own buoyancy. Once the structure is on location, it is made to sink or is lowered to the bottom of the ocean and anchored there by piling driven into the ocean floor. The remaining portion of the permanent platform is erected on top of the understructure, which remaining portion might be a drilling derrick or a production facility, or both. Generally, along with the construction of a permanent platform there will be a simultaneous laying of one or more underwater pipelines to a shore side storage terminal.
The objective of the production platform and the pipeline is to receive the well stream; to separate the well stream into oil, gas, and water; and to transport the oil and gas to the shore terminal. At the shore terminal, the oil is stored and further transported by pipeline or ship to refineries where it can be processed into usable products. The gas is stored and further transported by pipeline to be used as fuel or as feedstocks for petrochemical processing plants. In some cases, the gas is liquefied and further transported by ships to be used as fuel.
In the conventional arrangement described above, to produce a marketable product requires the installation of a production platform, one or more underwater pipelines from the platform to the shore, a shore storage facility for the raw hydrocarbons, a process plant to convert the raw hydrocarbons to a finished product, and a means of transporting the product to market or to the consumer. This arrangement requires a large capital investment, and consequently the offshore hydrocarbon reservoir must be sufficiently large to justify the investment.
In some cases, the reservoir may have sufficient oil reserves to justify commercial production of the oil, and at the same time the gas quantities in the well stream may be insufficient to permit economic recovery of the gas. In such instances, the gas is flared at the oil production platform, or it may be reinjected into the well in an effort to improve the oil flow from the well.
In other cases, where the reservoir contains mostly gas, the size of the reservoir must be very large to permit economic recovery of the gas from offshore locations. Where the initial well stream tests show marginal or insufficient gas quantities present, the gas well will be plugged as noncommercial.
The developments in the last few years regarding the prices of hydrocarbons and the high rates of inflation worldwide have brought about a situation where there is increased demand for hydrocarbons and natural gas in particular, yet the costs of recovering these products have become so great that the size of the economic offshore reservoir has become gigantic. Thus, there is an increasing number of smaller wells which have respectable quantities of gas which cannot be brought to market profitably.
Accordingly, it is desirable to improve the profitability of the small or marginal offshore reservoirs by bringing the petrochemical process plant to the well site rather than bringing the hydrocarbons to the shore side plant. By processing the hydrocarbons at the well site, many of the costs of the traditional arrangement are eliminated. Notable among these are the pipeline costs.
In bringing the process plant to the well site, provisions must be made to accomplish all of the functions required in the production and processing operations involved in converting the well stream products to a usable refined product. These functions include: drilling of production wells to develop the reservoir; completion of the wells; normal production separation of the well stream into oil, gas, and water; petrochemical or mechanical processing of the gas or oil into a usable refined product; storage and handling of the product and fresh water needed in processing; transportation of the product to market; maintenance of the petrochemical plant; and servicing of the wells. Any offshore process facility must provide arrangements to perform all of these necessary functions.
The products which could be produced from an offshore hydrocarbon processing plant include any of the derivations from petroleum. Primary products from gas flow would include: liquid petroleum gas, liquid natural gas, ammonia, urea, and methanol. From oil, the products could include: carbon black, gasoline, naphtha, heating oil, and others.
There are a number of configurations that an offshore hydrocarbon processing plant could take. These include a fixed platform, a floating vessel, and a jack-up type platform.
The fixed platform arrangement would be limited to situations where the reservoir was large enough to amortize the plant investment. This size reservoir may be large enough to produce economically in the traditional manner. The fixed platform would pose problems in maintenance of the process plant, unless it was located close to an industrial area and situated in calm water. Typically, a petrochemical plant operates about 330 days per year and then has a 35 day "turnaround" period where major maintenance work is performed. The work would include renewing of catalysts in reactor vessels, cleaning and repair of heat exchangers, repair and replacement of valves and piping, etc. The turnaround period is one of very high labor intensity to perform the necessary work in the shortest possible time in order to get the plant back into operation. The fixed platform concept would require the turnaround to be accomplished at the offshore site, involving the use of floating derricks to lift the catalyst beds and holders out of the tower type reactors. Because of the close tolerances involved, any motion of the floating derrick would greatly complicate this operation. Also, it would require the presence of a large labor force at the platform, together with all the parts and services needed for a rapid turnaround. Thus, the fixed platform approach is limited to specific applications which would minimize the effects of these problems.
An alternate approach is to utilize a floating vessel as the support for the offshore process plant. This concept provides the mobility which lets the plant be utilized at several successive reservoirs and permits the plant turnaround to be performed at a shore side facility. It has disadvantages in providing the production and completion facilities under some circumstances, and the wave induced vessel motions, however, small, would seriously affect the performance of some petrochemical plant equipment.
The most attractive approach to offshore petrochemical processing plants is the jack-up type platform. The jack-up offers the advantages of both the fixed platform and the ship configurations without the accompanying disadvantages. There are many different jack-up platforms in use today. A typical jack-up platform has a buoyant hull that permits transporting of the platform to the well site and has separate support legs that project upwardly from the hull during transport. Once the platform has reached the desired location, the support legs are lowered into contact with the ocean floor and the platform is jacked up to a level above the surface of the water. Representative of this jack-up platform is Le Tourneau, U.S. Pat. No. 2,830,071. Another approach to jack-up offshore platforms is a multiple stage platform having an upper working platform that is jacked out of the water and a lower support platform functioning as a weight support and as bracing structure. Giblon, U.S. Pat. No. 3,797,265, discloses this type of mobile jack-up offshore platform.
Recently, yet another approach to jack-up drilling and production platforms for offshore gas production has been developed. This approach utilizes a buoyant jacket type support structure which requires no auxilliary buoyancy and is horizontally towable to the well site where it is securely anchored by piles to the ocean floor. A specially shaped barge/desk section outfitted for production is towed to the location after the support structure is anchored and is floated into position proximate the legs of the support structure. The deck section is then lifted clear of the water by jacking mechanisms and secured in place at the operating height. The jacks are then removed for use on another platform. All producing and processing equipment is installed and serviced at the onshore fabrication site before the barge/deck section leaves the shipyard. A jack-up production platform of this type is available from Raymond International, Inc. and is referred to as a "Tilt-up, Jack-up" platform. One unique feature of this jack-up is an arrangement whereby the production drilling is done through the legs. See Phares, U.S. Pat. No. 3,857,247.
The jack-up platforms in use today are all very suitable for use as exploration drilling vessels or as production platforms, but they are not completely satisfactory for use as a mobile offshore hydrocarbon process platform in that the apparatus must be versatile enough to provide arrangements for all of the functions previously enumerated. A jack-up of the type normally used for drilling presents serious problems involved in plant turnaround and in well servicing. The turnaround must be accomplished at the site or else the complete jack-up, including the legs and support structure, must be removed from the site to the shore industrial area. This would necessitate disruption of the well heads and risers for some plants, and in all instances, the transit time from the site to the shore facility would typically be prohibitively long.
In order to provide for all of the functions at the required site, the need exists for a jack-up platform configuration which would separate the legs and support structure from the operative facility. Thus, the legs and support structure could be put into place first, and then a drilling vessel docked thereon and lifted out of the water. When production drilling and well completion is finished, the structure can be jacked down, and the drilling vessel floated free and removed. Then a petrochemical process plant is floated over the structure and lifted out of the water. For maintenance, the process vessel is refloated and towed at relatively high speed to a shore industrial location. Meanwhile, the support structure could be used to lift a workover barge to perform any necessary well servicing functions. When the reservoir is depleted, the process barge and the legs and support structure can easily be moved to another site.
The Raymond International Tilt-up Jack-up embodies some of the features required of a mobile offshore platform in that it permits the buoyant platform to be separated from the jacket structure. However, it has a fixed jacket rather than a movable leg structure, and it does not provide an easy means to raise any barge which is not specifically configured to mate with the jacket legs. Also, the barge shape which mates with the legs is not necessarily shaped for rapid ocean towing which is required to save time on turnaround periods.
Another approach to offshore production work is a ship platform which utilizes a marine vessel having a process plant assembled on its deck and incorporating portions of a jacking mechanism to raise the ship above the water on support legs engaged by the jacking mechanism. An example of this type of offshore structure for production is the system disclosed by Sumner, U.S. Pat. Nos. 3,716,933 and 3,874,180. A system of the type disclosed in these patents offers certain advantages in mobility over the jack-up platform approach and is more stable than the ship used alone. However, difficulty could be experienced in dismantling the structure for movement to another location due to the fact that the vessel carries the jack holder, which requires that the support legs and structure attached to it be anchored from the ocean floor if the ship is required to be removed from the site. Upon the ship's return, it is necessary to re-anchor the support legs. The requirements of loosening and re-anchoring the structure prohibits the use of the ship until it is desired to move to a new well site.